A. Field of the Invention
The present invention relates to an image calibration method for a contact type scanner, especially to an automatic image calibration method that solves the problems of missing line or skew image occurred when the sensor chips of the contact image sensor are not in alignment.
B. Description of the Prior Art
A conventional contact image sensor module (hereinafter referred to as CIS module) consists of multiple sensor chips that are perfectly aligned in a bar-shape module without any space therebetween. For instance, a 600 dpi CIS module usually consists of 15 sensor chips. And each sensor chip can read 340 pixels. These sensor chips in the CIS module shall be in perfect alignment as illustrated in FIG. 1A. Accordingly, the image of a test chart read by these sensor chips will look like the image as illustrated in FIG. 2A. However, when the sensor chips are not well-aligned during assemble, the phenomena of missing lines or skew images may occur as illustrated in FIGS. 1B.about.1D. FIGS. 1B.about.1C illustrate the situations when the sensor chips are not well-aligned in a vertical direction. On the other hand, FIG. 1D shows the situation when the sensor chips are not tightly connected together in a horizontal direction.
As illustrated in FIGS. 1B.about.1C, when the sensor chips are not well-aligned in the vertical direction, the skew pixels can be detected from the horizontal lines or the slanting lines of the image of the test chart. On the other hand, when there is space between two neighboring sensor chips as illustrated in FIG. 1D, missing lines or pixels may be detectable from the edges of the slanting lines. In FIG. 2D, the missing pixels X, Y indicate the location where the sensor chips are not connected together tightly.
Although the quality of CIS modules from CIS manufactures is not always defected, a small or even invisible displacement of these sensor chips may affect the quality of images a lot. That explains why the image quality of the contact type scanners cannot be guaranteed. This has long been a problem for the scanner manufacturers because they have no control on the products what are from the third party.